This invention relates to a circuit for detecting the position of a signal recorded on a magnetic tape.
A conventional tape recorder is provided with a mechanical counter for detecting the rotation of a take-up shaft, the counter being used to obtain the recorded position of an aural signal. In this type of mechanical counter, however, unless one notes a predetermined counted value in advance, it is impossible to cause the tape to travel with high speed up to its corresponding recording position and to stop at this position. This is troublesome and is also insufficient in the sense that the tape travel should be stopped precisely at the predetermined recording position. Further, one cannot have knowledge concerning the tape length (the amount of signals recorded) from such counted value. This makes such a mechanical counter inconvenient, in the case of, for example, a dictating tape recorder, i.e. a so-called transcriber, to detect the recorded position of a signal in the tape. Recently, therefore, in order to solve such problems, at the time of recording for example a voice, a detection signal having a lower frequency than the aural signal is simultaneously recorded on the tape for a specified period of time and, at the cuing or reviewing mode during the fast forwarding operation or rewinding operation with the magnetic head in contact with the tape the detection signal is reproduced to detect the recorded position of such voice. The travel speed of the tape in such cuing or reviewing mode (hereinafter, referred to as "the detection mode") becomes several tens of times as high as that in the recording or playing-back mode. For this reason, the detection signal, in the playing-back mode when the tape is subjected to low-speed travel, is not reproduced since its frequency band is too low, but, in the detection mode the frequency of such detection signal increases up to a several tens of times higher frequency in accordance with an increase in the travel speed of the tape, so that such detection signal becomes reproducible. Since at this time the frequency of the aural signal becomes high so that the aural signal departs from the frequency band over which signal reproduction by the reproducing circuit is possible, it is impossible that such aural signal is reproduced. By counting the number of detection signals as above reproduced it thus becomes possible under the detection mode to stop the travel of the magnetic tape at a predetermined position. Simultaneously it also becomes possible, by such counting operation, to know the amount of recorded signals between the detection signals.
The speed of the tape at the recording and playing-back mode, however, in consideration of the amount of tape consumption and the aural amount of signals reproduced, is rather preferred to be changeable-over to two different speeds or so than kept constant. Usually, the tape speed in the detecting mode is set at the highest possible speed. Upon variation in the tape speed in the recording mode, therefore, the tape speed in the detection mode as taken relative to the former speed, that is, the magnification of increase in the frequency of the detection signal, varies. Thus, it is necessary to widen the frequency band of the reproducing circuit, to an appreciably large extent. This results not only in a complicated structure of the circuit involved but also in a decrease in the reproducing precision of the detection signal due to the mixing of noise which is attributable to such wide frequency band.